tx curve - update utxo selection

if the tx/block in the tx curve is very high, we might run out of utxos to use for creating tx; need to make sure we have enough utxos before generating the curve.

*edit*

more debugging showed that the cause of the following error wasn't lack of utxos but a flaw in the utxo selection process, so updated this issue accordingly
Shouldn't (after sending the most mature coinbase to ```com.utxo```) you try to put back again that coinbase you were trying to put into ```com.coinbase``` in the line above ?